1. Field of the Invention
The present invention generally relates to an optical filter mounted on the screen of a display adapted to emit linearly polarized light and, more particularly, to an optical filter disposed on a transparent touch panel or the like, which is mounted on a liquid crystal display apparatus.
2. Description of the Related Art
In recent years, among portable information devices such as a personal digital assistant (PDA), an electronic notebook (or pocketbook), a word processor, a notebook personal computer and a remote control device, a car navigation system, a bank terminal (for example, a cash dispenser), Internet Kiosks, and office automation (OA) equipment (for instance, point-of-sales (POS) terminals, facsimile (FAX) and copying machines), such devices of the type that utilize liquid crystals have been in increasing demand. Moreover, such devices of the type that further utilize transparent touch panels have been on the increase.
There have been the following three types of conventional transparent touch panels which are classified according to the kinds of materials of two composing layers thereof:
(.alpha.) (Film+Film) type; PA1 (.beta.) (Film+Glass) type; and PA1 (.gamma.) (Glass+Glass) type.
Method of preventing the reflection of external light (or ambient light) to be employed by each of such conventional transparent touch panels is determined in accordance with the material (film or glass) of an operating section (or layer). Thus, the conventional transparent touch panels will be described hereinbelow by being classified according to the material of the operating section thereof.
First, the case, in which the material of the operating section of a touch panel is a film (namely, is of the aforementioned type (.alpha.) or (.beta.)), will be described hereunder. As the method of preventing the reflection of external light, there have been developed a method of performing a non-glare treatment on the surface of a film of the operating section, and another method of applying AR (anti-reflective) coating to the surface of a film of the operating section.
(i) In the case of employing the method of performing the non-glare treatment on the surface of the film of the operating section, external light is dispersed or scattered by embossing, printing and applying a coating thereon and thus realizing a rugged surface of the film of the operating section.
(ii) Further, in the case of applying AR coating on the surface of the film of the operating section, the film is coated with a large number of layers respectively made of materials, such as SiO.sub.2 and MgF.sub.2 which are different in refractive index from one another. Thus, the reflection of external light in a visible range can be prevented. This utilizes the properties that the reflection of light occurs on the boundary surface of a substance and that when the light is incident from a substance having a small refractive index upon another substance having a large refractive index.
Next, the case, in which the material of the operating section is glass (namely, the aforementioned type (.gamma.), will be described hereinbelow. In this case, as the method of preventing the reflection of external light on the touch panel, there have been devised a method of etching the surface of glass of the operating section, a method of sticking a special film to the surface of a glass layer of the operating section, a method of applying AR coating to the surface of glass of the operating section and a method of affixing an ordinary (non-polarizing) filter to the surface of the operating section.
(iii) In the case of the method of etching the surface of a glass layer of the operating section, external light is dispersed by realizing an uneven surface of the glass layer by etching thereof. This method utilizes similar properties as utilized in the case (i).
(iv) In the case of sticking a special filter to the surface of the glass layer of the operating section, a protecting film, the surface of the glass layer of which undergoes an embossing and a non-glare treatment, is stuck thereto. This method is similar to a method of sticking a smoke sheet (or film) to a window of a vehicle.
(v) In the case of the method of applying AR coating to the surface of the glass layer of the operating section, the glass layer of the operating section is coated with a large number of stacked layers which are different in refractive index from one another, similarly as in the aforementioned case (ii). Thus, the reflection of light in the visible range is prevented.
(vi) In the case of affixing an ordinary filter to the surface of the operating section, external light reflected by the surface of the glass layer of the operating section is return light. Thus, if the transmittance (or transmissivity) of the ordinary non-polarizing filter is 40%, the intensity of the reflected light is obtained by 0.4.times.0.4=0.16, and is, therefore, attenuated.
In addition to the aforementioned methods, in the case that the material of the operating section (namely, in the aforementioned case (.gamma.), there have been developed a method of using a linearly polarizing plate, and a method of using a circularly polarizing plate which is a combination of a linearly polarizing plate and a phase difference plate.
(vii) Method of using the linearly polarizing plate is to prevent the reflection of external light by sticking the linearly polarizing plate to the surface of the glass layer of the operating section.
FIG. 11 schematically shows the configuration of a transparent touch panel using a linearly polarizing plate, which is mounted on the screen of a liquid crystal display device (LCD). Linearly polarizing plate 6 is stuck to the surface of a glass layer (an upper tin doped indium oxide (ITO) glass layer) of the operating section 4.
Transmissivity of the linearly polarizing plate 6 is usually not more than 45%. Thus, the intensity of the reflected light is obtained by o.45.times.0.45.apprxeq.0.2, namely, attenuated in such a manner as to become not more than 0.2.
(viii) Method of using the circularly polarizing plate, a combination of a linearly polarizing plate and a phase difference plate is stuck to the glass layer of the operating section. Thus, the reflection of external light is prevented.
FIG. 12 schematically shows the configuration of a transparent touch panel using a circularly polarizing plate, mounted on the screen of a liquid crystal device.
Circularly polarizing plate 8 is stuck to a glass layer (namely, an upper ITO glass layer) 4. Circularly polarizing plate 8 is a combination of a linearly polarizing plate 6 and a quarter-wave phase difference plate 7. Quarter-wave phase difference plate 7 is stuck onto the glass layer 4 of the operating section. Moreover, another linearly polarizing plate 6 is stuck thereon.
With such a configuration, external light is changed into linearly polarized light, the electric field vector of which lies along Y-axis, after passing through the linearly polarizing plate 6. Next, if this linearly polarized light is divided into a vibration in an optical axis Z of the phase difference plate and a vibration and a vibration in Z-direction of an orthogonal axis Y. these vibrations coincide with an extraordinary ray and an ordinary ray propagating a doubly refracting element or crystal, respectively. Thus, after passing through the phase difference plate 7, the phase difference between waves vibrating in Y-direction and Z-direction, respectively, is (1/4)-wavelength (namely, the linearly polarized light is changed into circularly polarized light). Part of light having passed through the circularly polarizing plate 8 is reflected by the surface of the touch panel. Then, the phase difference plate 7 causes again a phase difference of (1/4)-wavelength between the waves vibrating in Y-direction and Z-direction, respectively, to which return light acting as the reflected light is divided. Consequently, a total phase difference between the waves vibrating in Y-direction and Z-direction, respectively, into which the reflected light is divided, is (1/2)-wavelength after passing through the phase difference plate 6, in comparison with the case that there is no phase difference between those into which the initial external light being incident upon the plate 6 is divided.
Linearly polarized light, the plane of vibration of which lies in Z-direction, is synthesized from two light waves, the phase difference between which is (1/2)-wavelength. Plane of polarization of this linearly polarized light is orthogonal to Y-direction. Thus, the light having passed through the phase difference plate 7 cannot further pass through the linearly polarizing plate 6 (in the upward direction).
Thus, the reflection of external light can be prevented by the circularly polarizing plate 8.
The aforementioned conventional methods, however, have the following problems.
First, in the case of the conventional methods described in the foregoing descriptions (i), (iii) and (iv), the degree of non-glare effects is enhanced so as to realize a rugged surface of the film or the glass layer, an image is blurred owing to the relation between the pixel size and pitch of liquid crystal display device. If the degree of non-flare effects is further enhanced, the displaying surface of the device becomes clouded due to the brightness of external light. Consequently, an image becomes hard to observe.
Further, if the diffuse reflectance of the surface of the film or glass layer is increased, the transmissivity thereof is decreased. Moreover, it is necessary for enhancing the brightness of an image to raise the brightness of backlighting light in the liquid crystal display device. This, however, results in increase in the power consumption of the device and in reduction in the life of a source of backlighting. Furthermore, even if the reflection of external light can be prevented, light cannot be prevented from being reflected by the inside (namely, ITO layer and the glass layer) of the touch panel. Thus, the visibility cannot be improved.
Further, in the case of the conventional methods described in the aforementioned descriptions (ii) and (v), AR coating is applied to the surface of the film and glass layer of the operating section by coating the surface thereof with two to five layers of substances which are different in refractive index from one another. However, such methods have the following defects. Surfacial hardness of the coating is low, so that the surface of the film and glass layer is fragile. Additionally, some materials of the surface coating have low chemical resistance. Surface of the coating is easily affected by chemicals.
Further, in the case of these methods, even when the reflection of external light can be prevented, light reflected by the inside (namely, ITO layer and the glass layer) of the touch panel cannot be prevented. Thus, the visibility can not be enhanced.
Moreover, in the case of the method described in the foregoing description (vi), the reflectance can be restrained to some extent by sticking a non-polarizing filter to the surface of the film or glass layer. However, the transmissivity of the non-polarizing filter is low. This method, thus, has defects in that the lightness of an image is lowered and that the visibility is lowered in light environment.
In the case of the method described in the foregoing description (vii), the transmissivity corresponding to the linearly polarizing plate is 30 to 45%. Thus, an amount of the reflected light is decreased by a reduction in an amount of incident external light, which is caused by using a linearity polarizing plate. However, components of a signal sent from a liquid crystal display device are attenuated. Thus, the visibility of an image is degraded.
In the case of the method described in the foregoing description (viii), a circularly polarizing plate obtained by combining a linearly polarizing plate with a phase difference plate is used. Thus, the transmissivity of the circularly polarizing plate is decreased in such a manner as to be lower than that of the linearly polarizing device. The brightness or lightness of an image is decreased as the transmissivity is lowered. Moreover, the visibility in light environment is degraded. Furthermore, the hue of an image is changed by the circularly polarizing plate.
The present invention is accomplished to solve the aforementioned problems of the prior art.